java - java中带有静态字段的接口(interface)用于共享 'constants'

Question

我正在研究一些开源 Java 项目来了解 Java，并注意到其中很多都有某种“常量”接口(interface)。

例如，processing.org有一个名为 PConstants.java 的接口(interface)，以及大多数其他核心类都实现此接口(interface)。该接口(interface)充满了静态成员。这种方法是否有原因，或者这被认为是不好的做法？为什么不在有意义的地方使用枚举或静态类？

我发现使用接口(interface)来允许某种伪“全局变量”很奇怪。

public interface PConstants {

  // LOTS OF static fields...

  static public final int SHINE = 31;

  // emissive (by default kept black)
  static public final int ER = 32;
  static public final int EG = 33;
  static public final int EB = 34;

  // has this vertex been lit yet
  static public final int BEEN_LIT = 35;

  static public final int VERTEX_FIELD_COUNT = 36;


  // renderers known to processing.core

  static final String P2D    = "processing.core.PGraphics2D";
  static final String P3D    = "processing.core.PGraphics3D";
  static final String JAVA2D = "processing.core.PGraphicsJava2D";
  static final String OPENGL = "processing.opengl.PGraphicsOpenGL";
  static final String PDF    = "processing.pdf.PGraphicsPDF";
  static final String DXF    = "processing.dxf.RawDXF";


  // platform IDs for PApplet.platform

  static final int OTHER   = 0;
  static final int WINDOWS = 1;
  static final int MACOSX  = 2;
  static final int LINUX   = 3;

  static final String[] platformNames = {
    "other", "windows", "macosx", "linux"
  };

  // and on and on

}

Answer 1

这通常被认为是不好的做法。问题在于常量是实现类的公共(public)“接口(interface)”(因为需要更好的词)的一部分。这意味着实现类将所有这些值发布到外部类，即使它们仅在内部需要。常量在整个代码中激增。一个例子是 SwingConstants Swing 中的接口(interface)，它由数十个类实现，所有这些类都“重新导出”其常量(甚至是它们不使用的常量)作为自己的常量。

但不要只相信我的话，Josh Bloch also says这很糟糕:

The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class's exported API. It is of no consequence to the users of a class that the class implements a constant interface. In fact, it may even confuse them. Worse, it represents a commitment: if in a future release the class is modified so that it no longer needs to use the constants, it still must implement the interface to ensure binary compatibility. If a nonfinal class implements a constant interface, all of its subclasses will have their namespaces polluted by the constants in the interface.

枚举可能是更好的方法。或者您可以简单地将常量作为公共(public)静态字段放在无法实例化的类中。这允许另一个类访问它们，而不会污染它自己的 API。